The present invention relates to a residual echo suppressor for an echo canceller, which effectively prevents echo disturbances in long delay telephone circuits.
Long distance telephone facilities usually comprise four-wire transmission circuits between central switching offices at different local exchange areas, and two-wire circuits within each exchange area connecting individual subscribers with the switching office. A call between subscribers in different exchange areas is carried over a two-wire circuit in each of the areas and a four-wire circuit between the areas, with conversion of speech energy between the two and the four-wire circuits being effected by hybrid circuits. Ideally, the hybrid circuit input ports perfectly match the impedances of the two and four-wire circuits, and its balance network impedance perfectly matches the impedance of the two-wire circuit, so that signals transmitted from one exchange area to the other will not be reflected or returned to the one area as echo. Unfortunately, due to impedance differences which inherently exist between different two and four-wire circuits, and because impedances must be matched at each frequency in the voice band, it is virtually impossible for a given hybrid circuit to perfectly match the impedances of any particular two and four-wire transmission circuit. Echo is, therefore, characteristically part of a long distance telephone system.
Although undesirable, echo is tolerable in a telephone system so long as the time delay in the echo path is relatively short, for example shorter than 40 milliseconds. However, longer echo delays can prove to be utterly confusing to a far end speaker, and to reduce the same to a tolerable level an echo canceller may be provided toward each end of the path to cancel the echo that would otherwise return to the far end speaker. As is known, echo cancellers monitor the signal on the receive channel of a four-wire transmission circuit and generate an echo estimate of the actual echo that is expected to return over the transmit channel. The echo estimate is then subtracted from the echo that occurs on the transmit channel to remove or at least reduce the echo.
Ideally, the echo estimate generated by the echo canceller is identical to the echo occurring on the transmit channel, so that upon its substraction from the echo signal a complete cancellation of echo results. In practice, however, although the echo estimate may be a good approximation of the actual echo, it is usually not possible through cancellation to reduce the echo signal by more than 30 dB, so that residual echo remains. Consequently, additional control is required to eliminate the residual echo in order to achieve an acceptable level of performance in the telephone system.
One known technique contemplated by the prior art for dealing with residual echo has been termed "adaptive center clipping". This is a suppression technique based on a comparison of the time-averaged speech energy components on the transmit and receive paths, wherein suppression is enacted in the event of receive speech energy being greater than transmit speech energy. In an analog approach, a variable threshold amplifier receives the residual signal from the echo canceller and creates a squelching action to remove or clip center portions of the signal which have a level less than a reverse bias voltage. The reverse bias voltage is usually made to vary as a function of the time averaged level of received speech, and as long as the residual echo signal is lower in level than the reverse bias voltage it is completely blocked from being transmitted.
Although adaptive center clipping is effective to remove residual echo levels below the threshold established by the reverse bias voltage, sporadic residual echo levels in excess of the threshold are nevertheless transmitted to the far end subscriber. In addition, cross-over distortion results from removing the center portions of speech signals the upper levels of which are greater than the threshold voltage. The effects of cross-over distortion may be decreased through preemphasis and deemphasis filtering and by removing adaptive center clipping whenever near end speech is detected through time averaged speech comparisons, but this requires a "break-in" time of 50 to 150 milliseconds and a "hang-over" time of 100 to 240 milliseconds which delay insertion and removal of adaptive center clipping.
In accordance with a known digitally implemented center clipping technique, residual echo is reduced by blocking from transmission a fixed number of the lesser significant bits of the digitally encoded signal when near end speech is absent, and by passing all of the bits when near end speech is present as determined by a double talk detector. Unfortunately, if double talk detection is done on an instantaneous speech sample comparison basis, false detection of double talk and passage of residual echo is encountered when a zero cross-over value on the receive channel signal is compared in magnitude with an echo signal which at that time has a nonzero value. This induces "choppiness" in the residual echo, allowing portions of it to pass through unsuppressed. Should the speech samples be time averaged prior to comparison in order to minimize false detection of double talk, it is then necessary to delay insertion and removal of residual echo suppression. Also, as in the analog approach a form of cross-over distortion results since only those portions of the digitally encoded signal which are greater than the "fixed threshold" are transmitted. Consequently, the conventional digital approach to residual echo suppression has the same disadvantages as the analog technique.